Floodplains of Rivers

Figure 1: Typical alluvial river valley (diagram by Mandy).

By Ken Zhao

Above and left: Figure 2 North Saskatchewan River at Terwillegar Park, Edmonton. A) Aerial view (satellite imagery from Google Earth). B) Crosssection (elevation data from Natural Resources Canada).

Foreword

When it comes to natural disasters, Alberta is very familiar with the impact it’s had on its communities. Fires have been responsible for the destruction of a variety of communities in the Regional Municipality of Wood Buffalo and other parts of the province for years. The 2016 Horse River wildfire contributed to an estimated $3.6 billion of damage and is considered one of Canada’s worst natural disasters.

Alberta is also no stranger to floods. Floods contributed to millions of dollars in property damage in Calgary, High River and Fort McMurray. One of the top three natural disasters is associated with flooding. The 2013 Southern Alberta floods resulted in approximately $1.72 billion in property damage. These disasters continue to remind us of how fragile we are and that we need to improve our emergency and flood management programs across Alberta.

We are seeing more infrastructure being introduced to limit and control fire and floods in high-risk communities. We also see improvements in floodproofing buildings. For this edition, we wanted to provide some background information about flood plains. As building envelope professionals, we’re always looking for ways to improve the resiliency and sustainability of buildings. Understanding the surrounding environment is an important step to getting there. We were fortunate to have a few stories about the devastating events that have occurred and feature building envelop ideas that contribute to resilient buildings located in flood plains. We would like to thank those that came forward with the great stories and articles. In today’s fast paced world finding time to share our experiences is more important than ever. We hope you enjoy the stories and encourage you to send in your ideas for future publications. — Jamie Murphy A floodplain is the normally dryland adjoining alluvial rivers, over which water from the river flows at times of high discharge. Alluvial rivers typically consist of a main channel, floodplains, hillslopes at the formation of the

valley’s side, and terraces (the landscape elevated relative to the modern river), as illustrated in Figure 1. Floodplains are the areas that stretch from the banks of a river to the base of the enclosing valley walls. As described in the Encyclopedia of Geomorphology by Andrew Goudie (2004) , floodplains are formed by inchannel lateral and overbank vertical accretion processes. Lateral accretion takes place within the channel where the river meanders. As the channel migrates laterally by bank erosion on one side, deposition occurs on the side forming a bar, which grows laterally towards the direction of migration and increases in height. Over time, the bar is built up to a level close to bank elevations. When the river receives more water than can be accommodated by the main channel during high runoff seasons, excess water will flow over the banks and sediment to the overbank area, which will result in vertical accretion in the floodplain. For any river, the presence of the floodplains and their extent depend on many factors, including nature of floods, type of river valley and its erodibility. Figure 2 shows a meandering pattern of the North Saskatchewan River at Terwillegar Park, Edmonton and the shape of the river valley cross section. The main river channel normally carries low to average flows. As the inflow increases due to high runoff and exceeds the channel capacity, water will spill onto floodplains, and the rate of river level rise will decrease. The flow of water on the floodplain is shallower and slower than that in the channel as the floodplain offers a greater flow width and higher surface roughness if vegetated. As such, floodplains provide a river more room to convey floodwater and to attenuate flood waves. If floodplains are occupied (e.g. due to development) restricting flows in the channel, the river would flow higher and faster.

The extent of floodplain inundation depends in part on the flood magnitude, which is usually defined by the return period. In Alberta, the minimum design standard is the 100-year flood, of which the magnitude has a one per cent chance of being equalled or exceeded in any given year. Detailed hydrology and hydraulic studies are usually required to delineate the 100-year floodplain. The Government of Alberta manages the production of flood studies and maps under the Flood Hazard Identification Program. Flood inundation maps have been produced for many municipalities across the province (available at https:// floods.alberta.ca/). These maps are intended to depict flood risks for a community and to inform local land use planning decisions, flood mitigation projects, and emergency response planning. The area of land that will be flooded during the 100-year design flood is defined as the flood hazard area, which is typically divided into two zones: the floodway and the flood fringe. The floodway typically incudes the main channel and a portion of the floodplain where flows are considered being deep, fast, and destructive. The flood fringe is the floodplain area outside of floodway where the flow is generally shallower (less than one meter deep) and slower (less than one meter per second in speed). In Alberta, new development in the flood fringe may be permitted but not in the floodway. While floodplains are often flood hazard areas, they provide a river more room to attenuate and store flood flows and thereby reduce the flood risk of surrounding and downstream areas. Floodplains also provide other benefits of economic, social, and environmental value. They improve water quality by retaining excess sediment and nutrients and provide essential habitat for wildlife. Deposition of nutrient-rich silt and sediment often makes floodplains fertile agricultural areas. Mr. Can Hua (Ken) Zhao, Ph.D., P.Eng. is a hydrotechnical engineer and principal with Northwest Hydraulic Consultants (NHC). Ken can be reached at kzhao@nhcweb.com. n